The distinction between clean and unclean animals appears first in Gen. vii. 2-3, 8, where it is said that Noah took into the ark seven and seven, male and female, of all kinds of clean beasts and fowls, and two and two, male and female, of all kinds of beasts and fowls that are not clean. Again, Gen. viii. 20 says that after the flood Noah "took of every clean beast and of every clean fowl, and offered burnt offerings on the altar that he had built to the Lord." It seems that in the mind of this writer the distinction between clean and unclean animals was intended for sacrifices only; for in the following chapter he makes God say: "Everything that moveth shall be food for you" (Gen. ix. 3). In Leviticus (xi. 1-47) and Deuteronomy (xiv. 1-20), however, the distinction between "clean" and "unclean" is made the foundation of a food-law: "This is the law . . . to make a difference between the clean and the unclean, and between the living thing that may be eaten and the living thing that may not be eaten" (Lev. xi. 46-47). The permitted food is called "clean," "pure" (, ṭahor): the forbidden food is not simply not clean, but is positively unclean, polluted, impure (, ṭame), "an abominanation" (, sheḳeẓ). The terminology "clean and unclean" in the food-law has to a certain extent a different implication from that borne by the same terms as used in the sacrificial law (see Sacrifice).

The clean animals were:

(1) All quadrupeds that chew the cud and also divide the hoof (Lev. xi. 3; Deut. xiv. 6); for instance, the ox, the sheep, the goat (i.e., the sacrificial animals), the hart and the gazel, the roebuck, the wild goat, the pygarg, the antelope, and the chamois(Deut. xiv. 4-5). Among other forbidden animals, the camel, the rock-badger (see Coney), the hare, and the swine were excluded by name (Lev. xi. 4-7; Deut. xiv. 7-8), probably because used as food or for sacrifice by the neighboring tribes.

(3) Birds. Here the Law proceeds by way of elimination. From the rather lengthy list of forbidden birds (Lev. xi. 13-19; Deut. xiv. 11-18) it may be concluded that all the birds of prey and most of the water-fowl were considered unclean. The bat closes the list.

(4) The winged creeping things "that go upon all four" which "have legs above their feet to leap withal," of which four kinds of locusts are named (Lev. xi. 21-22). All the other creeping things (see Animals) are most emphatically and repeatedly forbidden and held up as the greatest abomination (Lev. xi. 20, 31-38, 42-43). A list of creeping things to be avoided includes the weasel, the mouse, four kinds of lizards, and the chameleon (Lev. xi. 29-30).

Restrictions were also placed on the use of the flesh of clean animals: it was forbidden to eat it when the animal had been torn in the field by a carnivorous beast (Ex. xxii. 30), or when it had died a natural death, or had been carried off by disease (Deut. xiv. 21). Although, however, the use of such meats rendered people unclean, strictly speaking, their prohibition belongs to the law concerning Blood.

For the distinction between clean and unclean animals various origins have been suggested; though few of them seem to have fully satisfied any one but their own originators. Omitting the most ancient ones (Origen, "Contra Celsum," iv. 93; ed. Migne, xi., col. 1171; Theodoret, on Lev. ix. 1, ed. Migne, lxxx., col. 299, and others, analyzed in Vigouroux, "Dict. de la Bible," i. 615 et seq.), only the most popular ones in our own day need be mentioned. According to Grotius, on Lev. xi. 3; Spencer, "De Leg. Hebr. Rit." i. 7, 2; S. D. Michaelis, "Mosaisches Recht," iv., § 220, etc., the distinction between clean and unclean animals is based on hygiene: it is a sanitary law. According to others, the law was a national one, intended to separate Israel from the neighboring nations, Arabians, Canaanites, and Egyptians (Ewald, "Antiq. of Israel," pp. 144 et seq.), and partly a sanitary one (Rosenmüller, "Scholia in Vetus Testamentum"—Leviticus). According to Keil, "Handbuch der Biblischen Archäologie," pp. 492 et seq., the law is a religious one, intended to deter men from the vices and sins of which certain animals are the symbols, which view is a mere variation of the allegorical interpretation proposed by Philo ("De Concupiscentia," 5-10).

Of these explanations the first two have been refuted by Sommer in his "Biblische Abhandlungen," i. 187-193; Keil's opinion has been opposed by Nowack, "Lehrbuch der Biblischen Archäologie," i. 117, and others. The most popular theory at the present day is perhaps that offered by the late W. Robertson Smith, in his article "Animal Worship and Animal Tribes Among the Ancient Arabs" ("Journal of Philology," 1880), according to which the unclean animals were forbidden because they were totems of the primitive clans of Israel. This theory has been accepted by Cheyne ("Isaiah," i. 99; ii. 123-124, 303) and Stade ("Gesch. Israels," i. 408), but by Dillmann is either entirely and without discussion rejected ("Genesis," p. 382), or restricted to the prehistoric times of Israel, as being a survival of the old totem-worship and totem-clan organization, resembling in historic times the case of the horse in England, which anthropologists say is not eaten because it was once sacred to Odin, and thus tabooed (Joseph Jacobs in his "Studies in Biblical Archeol." p. 89, and similarly Salomon Reinach, "Les Interdictions Alimentaires et la Loi Mosaïque," in "Rev. Etudes Juives," xli. 144). See Blood; Food; and Totemism.

The distinctions between clean and unclean animals, as described in the Scriptures, are more fully drawn in the Halakah. To chew the cud and to have split hoofs (Lev. xi. 3) are the marks of the clean tame quadruped ("behemah"), and the Talmudic traditions add that an animal without upper teeth always chews the cud and has split hoofs (see Aristotle, "Natural History," ix. 50), the only exceptions being the hare and the rabbit, which, in spite of having upper teeth, chew the cud and have split hoofs, and the camel, which has, in place of upper teeth, an incisor on each side (). Even the meat of the clean and the unclean animals can be distinguished. The meat of the former below the hipbones can be torn lengthwise as well as across, which, among unclean animals, is only possible with the flesh of the wild ass. These differences apply also to clean wild animals () as against unclean wild animals (). In order, however, to distinguish clean wild from clean tame animals attention must particularly be paid to the horns. The horns of the former must be forked, or, if not forked, they must be clear of splinters, notched with scales, and be ("round"), or, as others read, ("pointed"). It is important to distinguish the clean wild animals from the clean tame animals, because the tallow of the former may be used, while that of the latter is forbidden, and the blood of the clean wild animal must be covered up (Lev. xvii. 13), which is not the case with that of other animals (Ḥul. 59a, b).

It was hard for the rabbinical authorities to distinguish clean from unclean birds, as the Scripture (Lev. xi. 13-19) enumerates only the birds which shall not be eaten, without giving any of the marks which distinguish them from the clean birds. This is all the more important as the names of some of the birds mentioned in the Scriptures are followed by the word "lemino" or "leminehu"—i.e., "after its kind"—and it is therefore necessary to recognize certain fixed distinguishing characteristics. The following rules are fixed by the Talmud, by which a clean bird may be distinguished. It must not be a bird of prey; it must have a front toe, if that be the meaning of ; but according to most explanations the hind toe is meant. Although most birds of prey have the hind toe, the toes of the clean bird are so divided that the three front toes are on one side and the hind toes on the other, while the unclean bird spreads his toes so that two toes are on each side; or if it has five toes, three will be on one side and two on the other (compare Rashi to Ḥul. 59a, and Nissim b. Reuben on the Mishnah to this passage).

The clean birds, furthermore, have craws, and their stomachs have a double skin which can easily be separated. They catch food thrown into the air, but will lay it upon the ground and tear it with their bills before eating it. If a morsel be thrown to an unclean bird it will catch it in the air and swallow it, or it will hold it on the ground with one foot, while tearing off pieces with its bill (Ḥul. 59a, 61a, 63a). As this distinction is not found in Scripture, opinions differedgreatly during and since Talmudic times. According to the Talmud (Ḥul. 62a, 63b), only the twenty-four kinds of birds mentioned in Scripture are actually forbidden. If certain birds are positively known as not belonging to these, no further investigation as to characteristic signs is necessary, and they may be eaten. The marks of distinction are laid down only for cases in which there is doubt whether the species is clean or unclean. Authorities, especially in Germany, would only permit the eating of such kinds as have always been eaten (). Accordingly some birds are permitted to be eaten in certain countries, but not in others. There are many controversies in the casuistic literature concerning this matter. Menahem Mendel Krochmal ("Ẓemaḥ Ẓedeḳ," No. 29), for instance, declares the wild goose forbidden, while Eybeschütz ("Kereti u-Peleti," § 82) permits it. When the turkey was brought to Europe Isaiah Horwitz forbade it to be eaten; and although his opinion did not prevail, his descendants refrain from eating it even to-day.

In regard to clean and unclean fishes the authorities of the Talmud have also made some additions to the regulations in the Scriptures. While it is stated in Lev. xi. 9 that only those fishes are to be considered clean which have scales and fins, the Mishnah (Niddah vi. 9) declares that all fishes with scales have, doubtless, fins also. According to this all fishes having scales but no fins may be eaten, as under that opinion it may be taken for granted that all scaly fishes have fins; apparent exceptions are accounted for by the supposition that sometimes fins are so small or rudimentary that they can not be distinguished. On the other hand, a fish with fins may be without scales and thus be unclean. The formation of the spinal cord and head also affords means of distinction. The clean fishes () have a perfect spinal column, and a head of a more or less flat projection; the unclean fishes have no spinal bone, and their heads end in a point ('Ab. Zarah 39b, 40a). There is a difference in the form of the bladder and roe in clean and unclean fishes. In clean fishes the bladder is blunt at one end and pointed at the other; while the unclean have the ends either both blunt or both pointed. Whether these marks can be depended on when the scales and fins are absent, or when the actual condition can no longer be positively ascertained, has been much discussed by old authorities (compare Jacob b. Asher, Ṭur Yoreh De'ah, 83). As a "cause célèbre" of modern times may be mentioned the controversy of Aaron Chorin with many Orthodox rabbis concerning the eating of sturgeon, which Chorin declared permissible, contrary to all former usage.

Concerning the use of the four kinds of locust permitted in the Scriptures (Lev. xi. 21-22) the Mishnah (Ḥul. iii. 8) says that a clean locust must have four feet, two of which are for jumping, and four wings, which must be long and broad enough to cover the whole body. But it is still subject to the restriction that, to be eaten, it must belong to the species , and there must be a reliable tradition recognizing it as eatable. Later authorities (compare Samuel b. David ha-Levi on Yoreh De'ah, 85) forbid its use entirely. Very rigorous are the rules set down by the Rabbis concerning the eating of "creeping things which crawl upon the ground" (Lev. xi. 41). According to the Rabbis only such "worms" are permitted for food as do not live in an isolated condition, but are found only in other substances; for instance, the maggots in meat, fruit, fish, drinkingwater, etc. But even in such cases the eating is forbidden if the worms have been removed from the place in which they originated, or if they have left that place and returned to it, thereby practically excluding all worm-eaten food (Ḥul. 67a, b). The conditions concerning the enforcement of these rules are very complicated (compare Yoreh De'ah, 84), but it may suffice to point out the following: Fruit and vegetables must be thoroughly examined before use to see whether they contain worms, and Orthodox families pay strict attention to the fact that should the food, after cooking, be shown to have been worm-eaten, it is not fit for consumption (compare Danzig, "Ḥokmat Adam," pp. 35, 22).

There was much speculation as to the reasons why certain species of animals should be allowed as food and others forbidden. In the Letter of Aristeas (lines 144-154) it is explained at length that "these laws have been given for justice' sake to awake pious thoughts and to form the character." It is especially emphasized that birds of prey have been forbidden, to teach that man shall practise justice; and not, depending upon his own strength, do injury to others. The marks which distinguish the clean animal are allegorically explained, as shown in the following instance: To have two feet and split hoofs signifies that all actions shall be taken with consideration of the right and wrong (compare Allegorical Interpretation). The martyr Eleazar, in IV Macc. v. 25, answers the king, who ridicules the laws forbidding unclean animals, "Whatever is congenial to our soul He permits us to eat; the use of obnoxious meats He forbade us." In this is apparently expressed the same idea which is stated later on by Ẓarẓa in the words: "All these things are forbidden, because they deprave the blood and make it susceptible to many diseases; they pollute the body and the soul" (Meḳor Ḥayyim, "Tazria'," beginning).

The prolix allegories of Philo concerning the clean and unclean animals (compare "De Agricultura Noe," xxv.-xxxi.) have been far surpassed by the Church Fathers (Irenæus, "Adversus Hæreses," v. 8; Clemens Alexandrinus, "Pædagogus," iii.; Origen, Hom. 7 in Lev.; and many others), and for this reason in many Jewish circles no exposition of the law whatever would be heard. One should not say "The meat of the hog is obnoxious to me," but "I would and could eat it had not my Heavenly Father forbidden it" (Sifra, Ḳedoshim, end). In Talmudic-Midrashic literature no attempt is made to bring these laws nearer to human understanding. It was feared that much defining would endanger the observance of them, and all were satisfied "that they are things the use of which the Torah forbids" (Tanḥuma, Lev. ed. Buber, Shemini, iii. 29), although they were not capable of explanation.Beginning with Saadia, the Jewish commentators started to explain the Biblical laws either rationalistically or mystically. It is remarkable that Saadia's theory bears great resemblance to the modern theory of totemism. He asserts, namely, that some animals which were worshiped as divine were declared eatable as a protest against that worship, and for the same reason others were declared unclean ("Kitab al-Amanat Wal-I'tiḳadat," 117, bottom; Hebrew translation, iii. 2; ed. Slucki, p. 61). Ibn Ezra is of the opinion that the flesh of unclean animals has been forbidden because it is impure and obnoxious, and the substance swallowed and digested goes into the flesh and blood of those who have eaten it (commentary to Lev. xi. 93; concerning other passages of lbn Ezra compare Ẓarẓa, l.c.).

Maimonides ("Moreh Nebukim," iii. 48) finds in these ordinances mainly sanitary, and partly esthetic, principles. Similar is the opinion of the great French exegete Samuel b. Meïr, in his commentary on Leviticus. Naḥmanides agrees only partly with these theories, and mentions only one sanitary reason concerning fishes. The clean, he argues, get nearer the surface of the water, and therefore possess a degree of heat which drives away too much humidity; while the fishes without fins and scales, which stay in the deep water, and especially those in swampy water, possess a degree of cold and humidity which acts mortally. It is different with the birds, which, with exception of the "peres" and "'ozniyyah," two species of eagles, are all birds of prey, the black and thick blood of which causes a marked inclination to cruelty. Concerning the quadrupeds, Naḥmanides wavers between ethical and sanitary reasons, and refers to non-Jewish physicians to maintain the objections to the flesh of the hog (commentary on Lev. ix. 13; compare his "Derasha," ed. Jellinek, p. 29). The explanations which Baḥya b. Asher (on Lev. xi.) gives concerning the forbidden animals are mainly taken from Naḥmanides. He adds the new explanation that this law is merely an expansion of the rules of the cult of sacrifice, so that many animals which can not be used for sacrifice shall not be eaten (idem, 163d. ed. Riva di Trento). Isaac Arama is especially opposed to sanitary reasons ("'Aḳedat Yiẓḥaḳ," part 60, ed. Pollak, iii. 33b), and acknowledges psychological and ethical motives only. "The unclean animals," says Arama, "cause coarseness and dulness of the soul." Arama, evidently referring to Abravanel, but without mentioning his name, gives other theories of Jewish scholars. In his remarkable polemic against the rationalistic explanation by Maimonides of the laws regulating food, Viterbo tries to show the untenableness of the sanitary grounds ("Ta'am Zeḳenim," ed. El. Ashkenazi, pp. 42-43).

Like the Jewish religious philosophers, the mystics have stated their speculations concerning the grounds of these laws. According to the cabalistic theory which makes the negative Sefirot the cause of the existence of evil in the world, the Zohar (Shemini, iii. 41b) explains that the unclean animals originate from some of these negative Sefirot, and therefore they are forbidden as food; but as with the arrival of the Messiah all will become purer and nobler, these animals will then be permitted as food (Yalḳ. Ḥadash, Liḳḳutim, 36, 79). In this manner the mystics explained the idea, expressed in Midrash Tehillim to cxlvi., that in the future God will declare the unclean animals clean. This Midrash caused Abravanel and other Jewish scholars much embarrassment (see Buber, ad loc.), so that several of them did not hesitate to declare it a Christian interpolation; but without reason, as similar opinions have been held and expressed in the remotest time (compare Antinomianism), and probably had their origin in pre-Christian times. Regarding the view taken by Reform rabbis and by modern Bible exegetes of clean and unclean animals, see Dietary Laws; Purity; Reform; Totemism.